1. Field of the Invention
The present invention relates to a multi-path monochromator and an optical spectrum analyzer using the multi-path monochromator.
2. Description of the Related Art
FIG. 2 shows an example of configuration of a multi-path monochromator (which is described in Japanese Patent Publication Tokukai Hei 8-145795). In FIG. 2, a reference numeral 1 represents an optical fiber. A reference numeral 7 represents a parabolic mirror. A reference numeral 4 represents a diffraction grating. Many grooves or gutters are formed on the surface of the diffraction grating 4 which outputs parallel lights at each wavelength in different angles. A reference numeral 5 represents a plane mirror. A reference numeral 8 represents an output slit. In the above-mentioned configuration, incident or input light supplied from the optical fiber 1 is converted into the parallel light which is launched into the diffraction grating 4.
An output light outputted from the diffraction grating 4 is returned back to a same path by the plane mirror 5 and is again supplied to the diffraction grating 4. The second time output light outputted from the diffraction grating 4 enters the parabolic mirror 7 to pass through the output slit 8 which is positioned at a focal point of the parabolic mirror 7. In the configuration, the wavelength λ1 of the light passing through the output slit 8 is given by a following Equation (referring to FIG. 7).mλ1=2·d·cos(θa/2)·sin θ1  (1)
where m represents diffraction degree, d represents a distance between grooves in the diffraction grating 4. θ a represents an angle between the incident light and the output light in the diffraction grating 4, θ1 represents an angle between the normal line of the diffraction grating 4 and a bisector of the angle between the incident light and the output light.
In addition, resolution of the wavelength passing through the output slit 8 is given by a following Equation (2) in the configuration.RB=d/(2·m·f)··S·cos β  (2)
where f represents a focal length of the parabolic mirror 8, S represents a width of the output slit 8, β represents an angle between the normal line of the diffraction grating 4 and the output light (the second time output light outputted from the diffraction grating 4 in the above-mentioned example).
The configuration is a two-path type monochromator in which twice diffractions occur by the diffraction grating 4. It is possible to obtain a high resolution in the configuration.
Problem 1:
The size of each parts is determined in accordance with the size of the parallel light beam which is supplied to each parts. In the parallel light, the beam diameter Φ is determined on the basis of an extensity angle θ0 of the incident light and a focal length f of a collimator which converts the incident light into the parallel light. The beam diameter Φ is given by Equation (3) (referring to FIG.8).Φ=2·f·tan(θ0/2)  (3)
In general, the extensity angle is constant in the incident light in case where the incident light is inputted to the monochromator, using the optical fiber 1. Therefore, the size of the parts in the monochromator is dependent on the focal length of the collimator which converts the incident light into the parallel light. In addition, it is possible to improve the resolution in the monochromator as the focal length becomes long in the collimator which is for focusing the output light on the output slit, as readily understood from the Equation (3).
Referring to FIG. 2, description will be made as regards of the problems of the prior art hereinafter.
According to the prior art, the same parts is used as the parabolic mirror 7 for converting the incident light into the parallel light and the parabolic mirror 7 for focusing the output light of the diffraction grating on the output slit 8. Accordingly, the beam diameter becomes large in the parallel light when the focal length is long in the parabolic mirror 7, in order to improve the resolution. As a result, the size of optical parts becomes large. When the optical parts become large in size, there are problems in which the optical parts become expensive and it is difficult to locate the optic parts in the monochromator.
In addition, it is impossible to receive the entire light of the beam by the optical parts when the optical parts are limited in size. The light loss in the monochromator increases. As a result, error may occur on measuring the intensity of the light when inputting a weak light.
Problem 2:
In addition, the same parts is used as the parabolic mirror 7 for converting the incident light into the parallel light and the parabolic mirror 7 for focusing the output light of the diffraction grating on the output slit 8 according to the prior art. It is necessary to separate the incident light from the output light passing through the output slit 8, in the monochromator.
In this event, it is necessary to input the output light of the optical fiber 1 that is the incident light of the parabolic mirror 7, to the parabolic mirror 7 outside the optical axis of the parabolic mirror 7, in case where the same parts is used is used as the parabolic mirror 7 for converting the incident light into the parallel light and the parabolic mirror 7 for focusing the output light of the diffraction grating on the output slit 8. Alternatively, it is necessary to input the output light of the diffraction grating 4 to the parabolic mirror 7 outside the optical axis of the parabolic mirror 7. Alternatively, it is necessary to input both of the above-mentioned output lights to the parabolic mirror 7 outside the optical axis of the parabolic mirror 7.
Description will be made with reference to FIG. 5. It is necessary to use an incident method in which the incident point of the optical fiber 1 does not exist on the central line of the parabolic mirror 7. Alternatively, it is necessary to use another incident method in which the output light is inclined with respect to Y-axis of the parabolic mirror 7.
In case of inputting the light to the parabolic mirror 7 outside the optical axis of the parabolic mirror 7, aberration occurs in the output light of the parabolic mirror 7. Inasmuch as it is impossible to focus the light on a point of the output slit 8 on the basis of the aberration, the image becomes large on the output slit 8 and it is impossible to obtain a desired resolution. As a result, there is a problem in which an optical characteristic becomes worse in the monochromator.
Problem 3:
In the monochromator, the wavelength of the light, which passes through the output slit 8, is defined by the Equation (1), as described above. More specifically, a rotational angle θ1 of the diffraction grating 7 through which the light having wavelength λ1 passes is given by:θ1=sin−1(m·λ1/2·d·cos θa)  (4)
Description will be made, referring to FIG. 6 which shows a top view of FIG. 2 for illustrating the prior art. When the rotational angle of the diffraction grating 7 is adjusted into θ1, the light having the wavelength λ1 is diffracted towards the parting angle θa and is returned back to the plane mirror 5 to pass through the output slit 8 through a same path.
In this event, there is a problem in which a wavelength component exists which is diffracted towards a direction similar to the incident direction from the parabolic mirror 7 in wavelengths (which are diffracted towards directions except for the parting angle θa) except for the wavelength λ1.
In general, the light, which is diffracted towards the direction similar to the incident direction, will be called a Littrow light. Referring to FIG. 10, description will be made as regards the Littrow light. Inasmuch as the Littrow light is returned back to the direction similar to the incident direction of the diffraction grating 4, the Littrow light is directly inputted the parabolic mirror 7 and passes through the output slit 8. When the rotational angle of the diffraction grating 4 is equal to θ1, the wavelength λx of the Littrow light which passes through the output slit 8 is given by a following Equation.λx=2·d·sin (θ1+θa/2)  (5)
In other words, the output slit 8 receives the light having the wavelength λ1 and the Littrow light having the wavelength λx at the same time. The light having the wavelength λ1 is a measuring object. As a result, it is impossible to measure the light having the wavelength λ1 that is the measuring object, in accuracy. In order t dissolve the above-mentioned problem, a following means is used in the prior art.
In this case, the light having the wavelength, which is t measuring object, will be called an ordinary light. In order to dissolve the above-mentioned problem, means is used which separates the Littrow light from the ordinary light on the output slit 8 at space. It is possible to separate the Littrow light from the ordinary light on the output slit 8 at space when slightly inclining the plane mirror 5. As a result, the reflection path of the Littrow light is different from the reflection path of the ordinary light and the Littrow light is separated from the ordinary light towards a height direction on the output slit 8.
However, it is necessary to input the ordinary light t the parabolic mirror 7 outside the optical axis of the parabolic mirror 7 inasmuch as the ordinary light is inclined in order to separate the Littrow light from the ordinary light. It is impossible to avoid the problem similar to the problem which is described in conjunction with the Problem 2.